The present invention, relates to loudspeakers.
The present invention, more particularly, relates to loudspeakers which utilize liquid suspension mechanisms for the voice coils incorporated into such loudspeakers.
Conventional loudspeakers commonly comprise a magnet assembly, and a non-magnetic annular frame, extending from the magnet assembly, to support the large end of a cone-shaped diaphragm. The small end of the diaphragm cone is attached to a voice coil that extends into an annular magnetic gap, provided in the aforementioned magnet assembly.
In order to accurately locate and suspend the voice coil within the magnetic gap, the voice coil is attached to the inner edge of an annular corrugated disk, formed of a flexible fabric material. The outer edge of the corrugated disk is attached to the frame at a point near the magnet assembly, so that the voice coil is partially supported, or suspended, by the corrugated disk. The corrugated disk is sometimes identified as a `spider`.
The magnet assembly commonly comprises a permanent annular magnet, polarized in the axial direction, and sandwiched between two magnetizable plates. One of the plates carries a cylindrical post that extends through the central space defined by the annular magnet, to form a cylindrical pole piece. The other plate has an annular opening, somewhat larger than the diameter of the post, or pole piece, such that an annular magnetic gap is formed between the post and the inner edge of the associated annular plate.
As previously mentioned, the voice coil extends into the magnetic gap. A variable A.C. signal applied to the voice coil produces a fluctuating magnetic force that interacts with the magnetic force produced by the permanent magnet assembly, whereby the voice coil is caused to oscillate axially within the magnetic gap. The cone-shaped diaphragm moves accordingly, to generate an audible output in the ambient space in front of the loud speaker.
The voice coil is designed to oscillate axially without experiencing other types of motion, such as rotation, moving obliquely to the axial direction, or moving in different directions, at different points, in the oscillation stroke. The corrugated disk is not entirely satisfactory as a coil suspension device in that the flexure resistance of the disk, or dimensional tolerances, can sometimes produce an oblique misalignment of the voice coil in the magnetic gap. The voice coil may move unevenly around its periphery, with a tendency to pitch or yaw, causing sound distortion. Should the voice coil scrape on the magnetic gap surfaces, the coil will experience premature failure.
The present invention relates to a magnetic liquid suspension means for locating and suspending the voice coil within the magnetic gap. The magnetic fluid can comprise a low viscosity oil, having microscopic particles of ironoxide, uniformly suspended therein. The oil-magnetic particle emulsion is adhered to the voice coil and to the magnetic gap surfaces, since the microscopic magnetic particles are magnetically attracted to the gap surfaces, by reason of the permanent magnetic field established across the magnetic gap. The microscopic, i.e., micron-sized, magnetic particles hold the liquid phase of the emulsion in the magnetic gap.
The use of a magnetic liquid as a suspension mechanism for the voice coil is advantageous, in that both the inner and outer surfaces of the voice coil are separated from the gap surfaces by the magnetic liquid. The voice coil is prevented from rubbing, or otherwise striking, the gap surfaces, i.e., the cylindrical post or the inner edge of the surrounding plate. Also, the magnetic liquid imposes minimal lateral restraint on the voice coil, so that the cone-shaped diaphragm, is relatively unrestrained. The cone moves according to the flexibility of its attachment to the annular frame. The large diameter end of the cone is attached to the frame by means of a molded elastomeric ring of semi-torroidal cross-section. If the elastomeric ring, and the frame, are manufactured with reasonably good precision, the cone will oscillate in essentially a true axial direction, so as to exert minimal constraint on the voice coil motion. The magnetic liquid located in the magnetic gap, will exert sufficient guidance on the voice coil, to ensure that the voice coil will move in a true axial direction.
One problem with the magnetic liquid voice coil suspension, is that the liquid has a tendency to be blown, or drawn, out of the magnetic gap, during operation, thereby depleting the quantity of liquid in contact with the voice coil. This phenomenon is due to the fact that oscillatory motion of the voice coil, produces momentary pressure changes in the atmosphere, near the end of the pole piece, and in the annular chamber, surrounding the pole piece.
As the cone-shaped diaphragm moves toward the magnetic gap, the pressure at the end of the pole piece is increased, thereby tending to blow some of the magnetic liquid out of the magnetic gap into the annular chamber surrounding the pole piece. As the cone-shaped diaphragm moves away from the magnetic gap, the annular chamber surrounding the pole piece experiences a slight volume decrease, with a corresponding slight negative pressure change. A pressure differential is set up along the length of the gap, such that some of the gap liquid may be drawn out of the gap, into the annular chamber. The present invention provides solutions, and mechanisms, for preventing the magnetic liquid from being drawn, or forced, out of the magnetic gap.